Since indium phosphide (InP) substrates have light emitting characteristics and allow higher electron mobility, such substrates are widely used in a semiconductor laser diode (LD), a light emitting diode (LED), a high-speed device, or the like. According to a semiconductor LD and an LED, after an epitaxial film is formed on an InP substrate, the PL intensity is measured. In this way, the light emitting performance can be inspected in a simple manner. It is preferable that this PL light emission intensity is stronger. Also in a high-speed device, it is problematic that a leakage occurs due to impurities at the interface between the InP substrate and the epitaxial film. Accordingly, the less the n-type or p-type impurities exist at the interface, the more the electrical characteristics are stabilized.
PTD 1 (Japanese Patent Laying-Open No. 2007-311490) discloses a technique of reducing impurities on the surface of a compound semiconductor substrate by forming the compound semiconductor substrate to have a surface roughness Rms of 0.2 nm or less.
PTD 2 (Japanese Patent Laying-Open No. 2010-248050) discloses a technique of suppressing deteriorations in PL characteristics and electrical characteristics, which occur when an epitaxial layer is formed on a substrate having a surface with an impurity concentration that is lowered by washing an indium phosphide substrate with a sulfuric acid/hydrogen peroxide and a phosphoric acid.